A very common mistake in T-SQL is trying to alter column definition trying to alter DEFAULT value (constraint) for that column. Probably, developers expect to have this because it is possible to assign default value for a column during table definition:
CREATE TABLE Table1 (
col1 INT NOT NULL DEFAULT (0),
col2 INT NOT NULL
Due to possibility to change column type from int to varchar(20) or null-ability with
ALTER TABLE Table1 ALTER COLUMN col2 VARCHAR(20) NULL
a lot of developers expect to be able to change a column default value with
ALTER TABLE Table1 ALTER COLUMN col1 DEFAULT (1)
Error message is displayed trying to execute this: "Incorrect syntax near the keyword 'DEFAULT'".
With a create table syntax mentioned earlier, SQL Server creates default constraint with auto generated name like "DF__Table1__col1__57DD0BE4" and stores default value in [text] column of syscomments table.
So, to change a default value for a column you should change a constraint definition and the only way to do that is to drop current constraint and create the new one:
ALTER TABLE table1 DROP CONSTRAINT DF__Table1__col1__57DD0BE4
ALTER TABLE table1 WITH NOCHECK ADD CONSTRAINT [Df_test_col1] DEFAULT (1) FOR col1
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